(1) Field of the Invention
The present invention relates to an active matrix liquid crystal display device, and in particular to the structure of a spacer for a liquid crystal display device of a latitudinal electric field type whereof the direction of an electric field to be applied to a liquid crystal parallels the phase boundary of a substrate.
(2) Description of the Related Arts
Conventionally, for a common active matrix liquid display device, a system (a longitudinal electric field type) is employed whereby a voltage is applied to two transparent substrates between which nematic liquid crystal is held, and the orientation of liquid crystal molecules is changed in consonance with the voltage applied to control the transmittance of light. The liquid crystal display device is constituted by an active matrix substrate, on which are formed a switching device and an electrode, and another substrate disposed opposite it, with liquid crystal sealed between these substrates and polarized plates located outside the substrates. When a voltage is applied to the electrodes formed on the substrate, the orientation of the liquid crystal molecules is altered and the transmittance of light is changed.
It is well known that the brightness of such a common liquid crystal display device changes because the optical activity of transmitted light varies, depending on the angle (the view field angle) at which a screen is viewed. For example, when a screen is positioned uptight and is viewed from the front, i.e., in the direction corresponding to the normal to surface line of the screen, an image having superior contrast is seen. However, when the screen is viewed from diagonally below the normal to surface direction, it is dark, and when it is viewed from further below, areas of contrasting brightness are inverted, i.e., a so-called gray scale inversion phenomenon occurs. Then, when the screen is viewed from diagonally above the normal to surface direction, it becomes whitish. These phenomena occur because the direction in which liquid crystals are aligned is determined by the display method, whereby optical activity is controlled by applying a longitudinal electric field (an electric field perpendicular to a substrate) to the liquid crystals and by aligning the liquid crystals in the direction of the electric field.
Recently, a latitudinal electric field display system has been drawing more attention as a system that can resolve the view field angle problem.
While a conventional liquid crystal display device controls the alignment of liquid crystals in the direction that runs parallel to a longitudinal electric field, which is perpendicular to a substrate, a latitudinal electric field display device controls the alignment of the liquid crystals in the direction of a latitudinal electric field that runs parallel to the substrate. Since in principle this system provides a wide view field angle and changes color tones only a little, it is considered to be the most effective, an improved system. As the latitudinal electric field display device is described in detail in Japanese Unexamined Patent Publication No. Hei 6-160878, no explanation for it will be given here.
Although compared with the longitudinal electric field type the latitudinal electric field liquid crystal display device has a wider view field angle, it requires light shielding portions, such as common electrodes, source electrodes and switching devices, and its pixel opening rate is slower than that of the longitudinal electric field display device. Multiple spacer particles are dispersed in order to form a cell gap, which affects the display characteristics of the latitudinal electric field type more than those of the longitudinal type.
Thus, a latitudinal electric field liquid crystal display device that does not employ spacer particles is disclosed in Japanese Unexamined Patent Publication No. Hei 6-214244. Since this liquid crystal display device is so designed that both a common electrode and a pixel electrode (a source electrode) are formed perpendicular to a substrate and are employed as spacers (struts), it does not require spacer particles.
For this liquid crystal display device, however, since it is difficult to form electrodes corresponding to a cell gap on a substrate and to perform a rubbing process on an alignment film formed on the surface of the substrate, devising such a display device is difficult.
A longitudinal electric field liquid crystal display device that requires no dispersion of spacer particles has been disclosed. In Japanese Unexamined Patent Publication No. Hei 7-281195, for example, is disclosed a liquid crystal display device wherein protrusions are formed on both an active matrix substrate and a substrate on which a color filter is formed, and the protrusions are brought into contact with each other to serve as spacer struts. This display device will be described while referring to included drawings. FIG. 8A is a plan view of the structure of one pixel of the liquid crystal display device, and FIG. 8B is a cross-sectional view taken along line VIIIB--VIIIB in FIG. 8A.
Gate wires 4 and drain wires 5 are formed on a transparent substrate 1 so that they intersect each other, and a switching device 8 using a thin film transistor (called a TFT) is arranged near each intersection. In the switching device 8, a gate insulating film 9 and a semiconductor layer 10 are overlaid on the gate wire 4, and the drain wire 5 and a source electrode 6 are formed via the semiconductor layer 10. Light shielding film 15 is laminated to form a protrusion on the thin film transistor, which serves as the switching device 8. Color filter material 16, 17 and 18 for three primary colors, R, G and B, are laminated to form another protrusion on a substrate, which serves as a color filter. An alignment film 20 is formed on each protrusion and a rubbing process is performed for the protrusions. Then, the substrates are overlapped so that contact is made with the protrusions and a cell gap is formed.
A film formation procedure is performed after a thin-film transistor is formed in order to form a thick light shield film on the thin film transistor. During this procedure, however, some deterioration of the characteristics of the thin film transistor occurs.
In addition, since spacer struts are located on the thin film transistor, external pressure is exerted on the thin film transistor, and this will adversely affect the transistor characteristics.
Another problem also arises. The shaded portions in FIG. 8A are display disabled areas, where light is shielded, and the non-shaded areas are display enabled areas. As is shown in FIG. 8A, the spacer struts on the thin-film transistor are located near display enabled areas, so that during the rubbing process, parts of the display areas are hidden by the spacer struts and can not be rubbed, which can precipitate display failures.